This invention relates in general to transducers, and more particularly to high resolution, high accuracy force transducers.
One prior art form of force or weight sensing utilizes a feedback approach using a movable coil in a fixed magnetic field. The coil is movable along a sensing axis and is driven by a current sufficient to maintain a fixed position along the sensing axis. In this configuration, the coil drive current provides a measure of forces applied to displace that coil. While this approach is generally effective, the force sensing configuration is relatively complex and correspondingly expensive.
Another form in the prior art is a strain gage load cell. In this form, however, the accuracy of the load cell is limited by hysteresis and creep of the strain gage sensor material, as well as that of bonding material for the sensor.
Applicant's aforementioned patents describe force transducers having a parallelogram structure utilizing a pair of "force summing" members at each side coupled through a pair of generally parallel, spaced-apart beam members. A relative displacement of the force summing members flexes the beam members. In the form shown, for example, in applicant's U.S. Pat. No. 4,448,085, the force summing members each support a "sensor" member, also formed of a dielectric material. The sensor members overlie one another in a spaced relationship, and carry conductive surfaces to form a capacitance gap. The change in the gap, and hence its capacitance, is directly proportional (a 1:1 correspondence) to the relative displacement d of the force summing members.
In applicant's U.S. Pat. No. 4,558,600, the sensor members are mounted on the beam members, rather than the force summing members, to provide a variation from the aforementioned 1:1 correspondence between the displacement and the gap change. As described there, the motion amplification at the sensor gap that is attainable with this construction is related to the height to width ratio of the transducer.
For certain applications, such as the use of force transducers as the central element of a high quality scale which transforms a force along one axis into a corresponding capacitance value, it is desirable to combine the advantages of the '085 parallelogram construction with the mechanical amplification attainable with the '600 construction while providing a transducer which is nevertheless highly compact.
It is therefore a principal object of the present invention to provide a high resolution, high accuracy force transducer that is both compact and has mechanical motion amplification.
A further principal object of this invention is to provide a force transducer with the foregoing advantages that also provides a push-pull mode of operation while maintaining a parallel plate relationship to provide good linearity, and hence accuracy.
A further object is to provide a force transducer with the foregoing advantages that is readily isolated from atmospheric changes, or can readily compensate for them.
Another object is to provide a force transducer with the foregoing advantages that can also shield the capacitance gap from stray electromagnetic radiation.
Still another object is to provide a force transducer with all of the foregoing advantages which can be manufactured at consistently high quality standards and at a favorable cost from known dielectric materials such as ceramic or quartz.